This invention relates generally to autopilots and more particularly to a marine autopilot having a roll compensation feature.
As is known in the art, a marine autopilot is used to maintain a ship, or vessel on a fixed course while the vessel encounters environmental variations such as changes in wind speed and direction and changes in sea conditions. Preferably, the vessel course is maintained with minimum intervention by the operator of the vessel. In particular, the autopilot adjusts the position of the vessel's rudder in order to compensate for effects caused by changes in, inter alia, waves, wind, currents, and vessel speed.
Some marine autopilots use a proportional plus integral plus derivative (PID) control law to maintain the vessel on a desired course (i.e. during course keeping operation) and a proportional plus derivative (PD) control law to change the course of the vessel (i.e. during course change operation). Such an autopilot provides an output signal, referred to hereinafter as a rudder control signal, which corresponds to a desired change in the position of the rudder. During course keeping operation, the rudder control signal is proportional to the summation of the following terms: an error signal (i.e. the difference between a desired course and the actual vessel heading), the time integral of the error signal, and the time rate of change of the error signal. Whereas, during course change operation, the rudder control signal is proportional to the summation of the error signal and the time rate of change of the error signal.
More particularly, each term in the conventional PID and PD control laws has a gain value associated therewith. The gain value associated with the error signal may be referred to generally as a proportional gain value, the gain value associated with the time integral of the error signal may be referred to generally as a trim value, and that associated with the time rate of change, or derivative of the error signal may be referred to generally as a counter rudder value. Thus, during course keeping operation for example, the rudder control signal is equivalent to K.sub.p e(t)+K.sub.d e(t)+K.sub.i .intg.e(t)dt, where e(t) is the error signal, K.sub.p is the proportional gain value, K.sub.i is the trim value, and K.sub.p is the counter rudder value.
In such a control system, the proportional term (i.e. K.sub.p e(t)) causes movement of the rudder to be proportional to the error signal. The derivative term (i.e. K.sub.d e(t)) provides damping in the sense that once the vessel yaws, the derivative term resists such motion, or angular velocity. In this way, the derivative term reduces overshoot of the vessel past its desired course. The integral term (i.e. K.sub.i .intg.e(t)dt) compensates for low frequency disturbances, such as wind, by providing a bias on the rudder position to offset the effect of such disturbances. Generally, the rudder control signal provided during course change operation is as described above with the exception that the integral term (i.e. K.sub.i .intg.e(t)dt) is nulled, or excluded.
With the above-described conventional control law, marine autopilots effectively compensate for deviations of a vessel from the desired course once such deviation has occurred and been sensed. In other words, the control system of the marine autopilot responds to deviations from the desired vessel course by appropriately changing the position of the vessel's rudder in accordance with the control law described above.